I think it depends on the battery type, some are more resistant than others. Just to be on a safer side tho, if the phone is a year or a year and a half old, you should stick to the 10%-15% range and charge it asap.
I'm not fully sure if they should be fully drained, however, try not to, not too often anyway. Be careful that if you charge before it's close to draining out too frequently, it will make a digital memory (basically, think that the 0% of the battery is not at 0% but at another % where you usually plug in your phone). That's why some phones shut off even when the battery reads (for example) 6%.
I have a bad habit of letting my phone discharge to 1%. There are two reasons for this. One, the phone is new so If I charge it fully on day 1, I'm still using it by the end of day 2, where the battery is almost dead. Two, the fact that my phone has quick charge and literally charges at a rate of 2% per minute in the first 30mins (quick charge slows down the higher the battery %).
Getting back to the original question, letting the device drain fully once in a while is not bad. You should not do it regularly though, I've seen on old devices it further ruins the battery, however, that is my own speculation.
In the end, it comes down to the age of the device or should I say the number of charges it has gone through. They are the measure that tells you by how many charge and discharge cycles will the battery start to perform worse or degrade in its quality to retain the charge. I think iPhones usually retain only 80% of their original capacity after 500 cycles and it is more or less the same for other phones.